A) SYSTEM CONCEPTS
It is estimated that the depleted oil reservoirs still contain well in excess of fifty percent (50%) of the original oil. The reference work, Fundamentals of Enhanced Oil Recovery, defines three different types of processes which enhance oil recovery from subterranean reservoir formations. The processes are classified as thermal processes, chemical processes and miscible displacement processes. This invention relates to a thermal process.
There are three types of thermal processes which have been commercially practiced in the recovery of oil from a reservoir formation. The first method is defined as steam stimulation which is also known as cyclic steam injection, steam soak or huff-n-puff. In this method, steam is injected into a producing well for about two to three weeks. Following this, the well is "shut in" for several days and then placed in production. The second process is the steam flooding process in which steam is injected into a number of injection wells while the oil is recovered from adjacent production wells. The last method is the "in situ" combustion method in which the oil reservoir is ignited through an injection well and continued injection of combustion air through the injection well drives the flame front propagation away from the injection well towards the production well. The propagation of the flame front can be somewhat controlled by the position of the injection well and then shifting the injection of combustion air from one injection well to another, etc.
All of these processes depend on or are based on the well-known fact that any heating of the oil remaining inside a reservoir decreases its viscosity and improves mobility of the oil. With increased mobility, additional oil recovery is possible. However, the commercial processes described, while highly practiced, have inherent defects which prevent full recovery or substantial recovery of the oil in the reservoir. In the steam stimulation method, the initial success of the method is quite good. However, only a relative small volume of the oil around the injection point will be drained from the reservoir. The rest of the reservoir is not affected and productivity decreases rapidly after the second or third injection try which is completely understandable from the way that heat is being applied to the reservoir.
In particular, when the steam penetrates the reservoir, it follows the path of least resistance and once the oil in this path is removed, subsequent injections simply follow the paths established in the initial injection. This changeling is commonly known as fingering and limits the effectiveness of the steam stimulation method. The steam flooding or injection method is somewhat more effective in the use of heat. This results simply because more of the reservoir is exposed to the steam than that in the steam stimulation method and thus more fingers arise. Once the fingers are formed, continued injection of the steam recovers very little if any, additional oil from the reservoir. Both steam stimulation and steam flooding methods are limited to wells which are not significantly deep because hydrostatic pressure must be lower than the critical steam pressure at 3,208 psig. Even with shallow wells and the use of the steam flooding method, the steam condenses as it is piped down the injection casing and once it is physically within the reservoir, condensation continues. In the process of condensation, steam generates latent heat increasing the sensible heat of the surrounding water heating the reservoir and reducing the viscosity of the oil. Again, the large losses in the steam piping are an inherent limitation in the efficient use of the system heat which affects all steam processes. For purposes of this invention, it is noted that inherent in the steam flooding process is the fact that hot water will exist in the reservoir upstream of the steam front. That is, hot water is produced by the steam front as it condenses and this hot water will initially be at the condensation temperature of the steam but the hot water will cool below this temperature as it gives up its heat to the reservoir formation.
In the in situ combustion process, the heat produced during combustion leads to an increase in temperature in the vicinity of the combustion process and in the formation of gas as a result of the thermal decomposition of oil. The process results in sudden steep temperature rises which leads to the thermal breakdown of the oil and this, in turn, results in reduced recovery and retention of a major portion of the oil within the reservoir in the form of carbon or coke. Again, the process is not well suited for applications where fingering and preferred flow paths have been established within the reservoir during earlier production, i.e. steam flooding or steam stimulation.
Within the prior art literature, Stahl U.S. Pat. No. 4,694,907 shows the use of hot water pumped through an injection well and then heated by an electrical down hole heater to produce steam for steam flooding. An orifice in the electrical down hole heater is said to compensate for the hydrostatic pressure developed in the hot water head so that steam can be formed in deep wells. Stahl uses an electrical down hole heater to generate steam and is cited to show conventional, electrically powered heaters.
Shu Canadian Patent 1,197,457 illustrates a process in which steam is initially injected through an injection well which is shut in until the pressure at the production well has dropped to a predetermined value. Hot water or low quality steam is then flooded into the oil reservoir and the production from the reservoir continues. Shu is believed pertinent because he shows that the adverse effects limiting production from the reservoir attributed to steam formed channels or fingers can be somewhat overcome by the use of hot water or low quality steam. However, Shu's process is obviously limited because the water or low quality steam injected into the well can only be heated to a relatively low fixed temperature, heat losses occur in transmission down the casing and the low temperature of the water in the reservoir cannot significantly heat the reservoir formation. Thus, the Shu process in the first instance is limited to shallow wells whereat steam can be initially formed and in the second instance is significantly limited in the sense that only a limited amount of heat can be inputted to the reservoir formation and this limits the oil recovery. In addition, Shu equates or teaches that low quality steam, a medium which can be compressed, can be interchanged with hot water, which is incompressible.
In a somewhat unrelated area, it is known to mine sulphur after salt has been removed from capped rock formations by means of the Frasch process. This process consists of heating water under pressure external to the formation to a temperature of about 325.degree. F. and then injecting the water into the capped rock of the dome. The super heated water flows out into the sulphur bearing deposit and when the temperature of the sulphur bearing formation reaches or exceeds the melting point of the sulphur, liquid sulphur flows to the bottom of the well whereat a differential pressure arrangement is used to carry off the molten sulphur. In Williams 4,157,847, a process is disclosed where additional water or steam is added to the water previously injected into the reservoir by means of an underground jet pump to improve the heat transfer capabilities of the "spent" previously heated water present in the formation. In Gil U.S. Pat. No. 3,614,986, a down hole electric heater is used for sulphur mining by the Frasch process at depths in excess of 2,000 feet. Gil's down hole heater heats the hot water back to its original surface temperature to compensate for the casing heat loss as the water travels from the surface to the sulphur bearing deposit. The basic concept is to use hot water heated at the surface and injected into the sulphur formation to liquify that portion of the sulphur which can be heated by the hot water before the hot water's heat is dissipated. The improvements relate to adding heat to the hot water previously injected into the formation. There is no heating of the formation.